In high speed operation of rotating parts, it is necessary to maintain such parts at a rate of rotation which provides for reliable and efficient results in accordance with predetermined levels. In the field of printing, for example, when the printing is accomplished by means of impact mechanism striking a character bearing member of the rotating type, such member must be rapidly moved into position for each and every character thereon to be struck by the impact mechanism. The character bearing member may, for example, be in the form of a drum or typewheel which is driven by a motor, the speed of such motor being maintained to make certain that the drum or typewheel is in the proper position at the instant that the impact mechanism is driven against the selected character thereon for printing of such character. It is, therefore, necessary that the speed of the motor driver for the character bearing member be adjusted so that such member can be positioned sufficiently fast to avoid printing a character too late in the cycle, while at the same time seeing that the motor is not driven faster than its rated speed. Of course, the motor speed control system should include provision for adjusting the speed of the motor so as to obtain a specified number of bearing member character positions in a predetermined time to meet the requirements of the operation.
A number of ways and means for controlling motor speeds have included both simple and complex systems. Representative of control systems in the prior art is U.S. Pat. No. 2,706,270, issued on Apr. 12, 1955 to F. G. Steele, which discloses a digital control system including a pair of pulse carrying channels for controlling the setting of a pair of reversing switches connected to rotate the armature of a motor, on the occurrence of each clock pulse, in the direction dictated by the information. U.S. Pat. No. 3,237,178, issued on Feb. 22, 1966 to A. G. Valentine, discloses control apparatus for a closed loop system having a phase detector, a reversible counter, flip-flop means or computer means, and a digital to analog converter or voltage switch means. Indicating means in the form of a voltmeter is connected to receive an analog output signal from the voltage switch means. U.S. Pat. No. 3,586,953, issued on June 22, 1971 to C. O. Markkanen et al., shows a stepper motor control system including an encoder which develops a coded signal representative of a plurality of shaft positions on either side of a reference position. Coded signals representative of the desired position of the motor and shaft position signals are applied to a comparator and logic circuit which develops output pulses to bring the coded signals and the position signals into coincidence. Another U.S. Pat. No. 3,648,141, issued on Mar. 7, 1972 to D. W. Scheer, shows a tape drive error-canceling system having a speed control circuit using comparator means for comparing a capstan speed responsive variable frequency signal with a reference frequency proportional to a desired capstan speed. A memory stores a separate speed correction signal for each of the capstan angular positions. The memory information is read concurrently with a sensing of each position and applied with the output signal from the comparator means to motor energizing means to control the capstan speed. U.S. Pat. No. 3,659,524, issued on May 2, 1972 to J. Beery et al., shows a printer control system including first drive means operated at one speed and second drive means operated at a slower speed, with means for detecting the presence of a document. The control means also synchronizes the operation of the print hammer, the print wheel and document movement responsive to speed of the document. And, U.S. Pat. No. 3,838,258, issued on Sept. 24, 1974 to D. J. Logan, discloses a control system for a step motor which includes a computer to control the energization of the motor in response to information supplied thereto. Motor increments are produced by the computer program without the aid of an external clock or feedback signals from the motor. The computer program includes one wait loop following each change in the output word to provide a predetermined delay preceding the next word change and to control the motor velocity.